Engine start/stop functions are used for reducing fuel consumption, as well as for reducing pollutant emissions of motor vehicles, configured with internal combustion engines and known from the practice. By way of these functions, internal combustion engines are shutdown with the aid of different vehicle concepts in appropriate operating states of the vehicle, wherein these functions are activated or de-activated, depending on the operating states of the various vehicle components, and the internal combustion engine of the vehicle is even shutdown during short vehicle standstills. Moreover, the internal combustion engines are again started up and/or shutdown when predefined startup criteria are present.
In order to prevent impairment of a conventional driving operation, due to an engine start/stop function, a quick startup process of the internal combustion engine and immediate buildup of power flow is necessary in the transmission mechanism of a vehicle when the driver of the vehicle requests to continue driving, especially when driving into heavy traffic and right-of-way roads. In conventionally designed automatic transmission mechanisms or automated manual transmissions, which are configured with shifting elements designed as wet-running disk clutches or disk brakes for engaging or disengaging different transmission steps of the transmission mechanism. The shifting elements are essentially supplied with the required actuating pressure, via a main transmission pump, only when the internal combustion engine is running. In this process, the startup pressure and/or actuating pressure of a shifting element are respectively adjusted, via at least one electric actuator.
From DE 10 2006 014 756.1, an apparatus is known which stores hydraulic fluid of a hydraulic system of a transmission mechanism, as well as a process for operating such an apparatus, in order to operate a vehicle running with an engine start/stop function with a desired spontaneity. The apparatus and/or hydraulic storage device is designed as a spring-loaded oil reservoir with an electromechanical detent mechanism.
The hydraulic storage device is filled by way of hydraulic pressure generated by a main transmission pump while the internal combustion engine is running, wherein a piston of the hydraulic storage device is positioned and maintained in position by a holding device as the filling level increases. When the internal combustion engine is shutdown and the drive of the main transmission pump is at zero, the hydraulic pressure in the hydraulic system of the transmission mechanism essentially decreases to zero. If there is a request from the engine start/stop function for starting of the internal combustion engine, the generation of power flow in the transmission mechanism is easily supported during the process of starting the internal combustion engine by forcing out the hydraulic fluid stored in the hydraulic storage device.
Furthermore, when the hydraulic system of the transmission mechanism reaches a depressurized state as a result of the shutdown of the internal combustion engine, the flow of electrical current to the electric actuators is set at a standby level, at which merely diagnostic functions may be performed via the actuators in the transmission mechanism and at which a rated input power of the transmission mechanism is minimized in order to considerably reduce the load on the electrical system of the vehicle. Moreover, the flow of electrical current to the actuators is changed upon a request from the engine start/stop function for starting the internal combustion engine and/or when starting the internal combustion engine is detected by the standby level, to an operating level, at which the actuators may each be energized to a required current value that is equivalent to the required operating state, such as engaging a gear ratio, a neutral state or a parking state. By changing the flow of electrical current to the actuators from the standby level to the operating level, the shifting elements being engaged to produce the desired operating state of the transmission mechanism are simultaneously acted on by the hydraulic system of the transmission mechanism and filled with hydraulic fluid.
However, it is disadvantageous in this process that additional actuating routines for feeding current to the actuators need to be provided for a minimum flow of electrical current to the actuators of a transmission mechanism in order to still be able to energize the actuators in this operating state of the transmission mechanism with at least the current values required for diagnostic functions.
From EP 1 069 346 B1, it is known to minimize the power load of the on-board electrical system of the vehicle when the internal combustion engine is shutdown by feeding current to the actuators of the transmission mechanism at a lower level than when the internal combustion engine is running.
With this known solution, it is disadvantageous that separate actuating sequences must be provided for implementing the operating mode of the transmission mechanism with a lower power requirement in addition to the actuating sequences for the flow of electrical current to the actuators during normal driving. These separate actuating sequences have, however, a complex design to ensure the required functionality and, therefore, require a corresponding storage space and also extend the actuation times of a transmission mechanism to an undesirable extent due to long calculating times.
Furthermore, as a result of the proposed separate flow of electrical current to the actuators when the internal combustion engine is shutdown, the shifting elements of the transmission mechanism have transmission capacities which, under certain circumstances, may lead to a locking of the drive output shaft of a vehicle and the output connected thereto when there is a system pressure in the hydraulic system that corresponds to the normal operation of the transmission mechanism. In order to prevent the function for implementing the minimum flow of electrical current to the actuators from being selected during normal driving, further security measures have to be provided which, however, entail additional efforts that further increase the manufacturing costs of a transmission mechanism.
It is therefore the object of the invention to provide a process for actuating a transmission mechanism with several shifting elements that can be acted on by a pressure source with hydraulic fluid such that the power required by a transmission mechanism can be reduced, when the internal combustion engine is shutdown, without additional efforts that could cause an increase of the manufacturing costs of the transmission mechanism in comparison with conventionally operated transmission mechanisms.